1. Field of the invention
The present invention concerns an active filter for filtering the current and optionally for improving the power factor of a single-phase overhead contact wire energized locomotive.
2. Description of the Prior Art
Telecommunication lines alongside railroad tracks may suffer interference due to the traction current flowing in the overhead contact wire and returning to the substation via the rail and the ground. This combination forms loops which generate a magnetic field and mainly causes interference on overhead conductors. The interference is worse if the conductors have no protective metal jacket.
In the absence of any protective measures, the interference voltages generated on low current conductors can be hazardous to personnel or to plant and can cause malfunctions. The railroad transmission system and the public network are susceptible to such interference over transverse distances up to two or three kilometers.
Single-phase overhead contact wire energized locomotives are equipped with AC/DC converters. On most French locomotives this converter is of the hybrid bridge circuit type. These rectifier bridge circuits operate in a natural switching mode which implies consumption of reactive power and generation of distortion which is retransmitted to the overhead contact wire.
Much work has been done on adding an input active filter to locomotives in order to reduce such interference, caused mainly by natural switching mode power supply bridge circuits. The principle is to provide a controlled current source in parallel with the overhead contact wire and able to inject into the latter currents of sufficient amplitude. The controllable current source may be based on a current loop voltage inverter connected to an auxiliary winding of the traction transformer, for example. The direct current supply to the inverter must have an amplitude greater than the peak secondary voltage.
As yet this work has not produced any practically usable devices. To overcome this shortcoming the present invention proposes an active filter with multiple voltage levels which is remarkably effective in filtering supply current harmonics.